In an ordinary cathode ray tube such as a television picture tube and so on, an electron gun is opposed to a phosphor screen and is extended backward along the direction substantially perpendicular to the phosphor screen with a result that a depth of a cathode ray tube envelope is considerably large. Whereas, a so-called flat-type cathode ray tube has been proposed that its electron gun is extended along the surface direction of the phosphor screen for making the tube envelope flat.
FIG. 1 schematically illustrates an example of such previously proposed flat-type cathode ray tube. In the figure, reference numeral 1 generally denotes a flat tube envelope, and in this flat tube envelope 1 are provided a target electrode 2 and a back electrode 3. These electrodes 2 and 3 are opposed to each other in the thickness direction of the flat tube envelope 1 (that is, the up-and-down-direction in the drawing). On the target electrode 2 is formed a phosphor screen 4. An electron gun 5 is provided on a plane which is parallel to and remote from the phosphor screen 4. Between the electron gun 5 and the phosphor screen 4 is provided a deflection coil 6 to deflect the electron beam emitted from the electron gun 5.
In such arrangement, to the target electrode 2 is applied a high voltage, for example, 10 KV and to the back electrode 3 is applied a voltage somewhat lower than the former, for example, 6 KV. As a result, the target electrode 2 and the back electrode 3 constitute an auxiliary deflection plate.
In such example, the target electrode 2 is extended to the position near the electron gun 5, surrounding the deflection region by the deflection coil 6, and the potential of the target electrode 2 is high, for example, 10 KV as compared with the potential of the back electrode 3, for example, 6 KV so that as shown in FIG. 2, within the flat tube envelope 1, particularly in the upper portion thereof is formed a diverging lens. Thus, the beam emitted from the electron gun 5 is diverged with a disadvantage that the circular-arc distortion is increased.
Moreover, in such example, since the target electrode 2 forming the auxiliary deflection plate is same in potential as a last grid 5a of the electron gun 5, the beam emitted from the electron gun 5 is never accelerated further. Therefore the power consumption of the cathode ray tube is urged to increase.